The present invention relates to multicellular honeycomb structures useful in applications of diesel engine exhaust filtration, and more particularly to porous ceramic filters of improved thermal conductivity and thermal shock resistance under conditions encountered in diesel exhaust systems.
Filters of diesel exhaust streams require a combination of high thermal shock resistance, excellent chemical and mechanical durability in harsh environments, and good filtration efficiency.
Cordierite monolith filters have been in use for heavy duty engine applications for nearly two decades. Benefits of these filters include minimal material and manufacturing costs. Recently, there has been an increase in demand of filters for passenger diesel cars due to tightening environmental pollution regulations. However, it appears that in passenger cars, especially under conditions of xe2x80x9cuncontrolledxe2x80x9d regeneration (i.e., where the onset of combustion coincides with, or is immediately followed by high oxygen content and low exhaust gas flow rates resulting in high temperature spikes) existing cordierite filters have shown tendency for failure. During an uncontrolled regeneration, the combustion of the soot (a reaction which is already highly exothermic) may produce temperature in the cordierite filter body of up to 1300xc2x0 C.-1500xc2x0 C. or even higher.
Even if the temperatures are not sufficient to damage the filter, they may be high enough to cause partial sintering to the surface of the filter walls of xe2x80x9cashxe2x80x9d particles, which consist of oxides such as those of calcium, zinc, magnesium, phosphorus, and sulfur, that are carried by the exhaust gas from burning of the engine lubricating oil, and oxides of metals such as iron and cerium that may be present as additives to the diesel fuel to aid in combustion of the soot. Additionally, oxide particles of iron, copper, and zinc may be present from wear of the engine components. These particles are not combustible and, therefore, are not removed during regeneration. Such sintered ash deposits may be difficult or impossible to remove from the filter during periodic maintenance, resulting in (i) a decrease in pore size and porosity and an increase in back pressure and loss in soot loading capacity, and (ii) formation of eutectics from reaction with the filter material which lowers the melting point of the underlying ceramic material.
As an alternative to cordierite monolith filters, silicon carbide (SiC) filters have emerged as the preferred choice. Benefits of these filters reside in SiC""s stable composition, high mechanical strength and high thermal conductivity. However, SiC also has high thermal expansion and poor thermal shock resistance which may result in cracking of the filter during regeneration cycles. Consequently, current SiC diesel particulate filters have to be designed in segmental configuration to minimize cracking during use, which translates into higher costs of manufacturing and production.
In view of the foregoing drawbacks in the prior art, an object of the present invention is to obviate the above-mentioned problems in diesel exhaust filters by providing a composite cordierite body of improved thermal and mechanical properties under conditions encountered in diesel exhaust systems. Cordierite ceramics provided in accordance with the invention may be made by conventional extrusion or other forming processes, so that ceramic products in any of the configurations presently utilized, but exhibiting improved thermal properties, may be provided.
Ceramic bodies provided in accordance with the invention comprise a non-oxide polycrystalline phase selected from the group consisting of carbides, nitrides and borides constituting 10-70% by weight of the body, and the remainder an oxide phase, preferably cordierite (magnesium aluminum silicate). Preferably, the non-oxide polycrystalline phase constitutes 10-50% by weight of the body, and more preferably 10-30% by weight of the body, is selected from the group consisting of polycrystalline silicon carbide and polycrystalline silicon nitride and has a particle aspect ratio of less than 3. In addition to cordierite, other suitable oxide phases include alkali aluminum silicates, such as lithium aluminum silicate and potassium aluminum silicate, alkaline earth aluminum silicates, such as calcium aluminum silicate and barium aluminum silicate.
Inventive structures are porous with an open porosity of at least 30%, preferably between 40% and 60%, and a mean a median pore size of at least 5 micrometers, preferably between 6 and 30 micrometers, and more preferably between 8 and 12 micrometers.
The inventive structures further exhibit a mean coefficient of thermal expansion from 22-1000xc2x0 C. of about 20-45xc3x9710xe2x88x927/xc2x0 C., and a four-point modulus of rupture as measured on a cellular bar of at least about 300 pounds per square inch (psi), preferably at least about 700 psi, and more preferably at least about 1000 psi.
The manufacture of a composite cordierite ceramic body in accordance with the invention comprises first providing a ceramic batch compounded from source materials for the cordierite phase and the non-oxide phase. For the cordierite phase suitable sources include reuse cordierite powder and conventional raw material comprising source material for MgO, Al2O3, and SiO2. For the non-oxide phase suitable sources include powdered silicon carbide and powdered silicon nitride. The batch is compounded in proportions which will yield, after firing, a mixed cordierite-non-oxide composition.
The compounded batch is next shaped into a green body, preferably a multicellular honeycomb structure, by a suitable ceramic forming process, preferably extrusion. In a preferred embodiment, the batch is blended with a suitable vehicle to form a plasticized mixture, and this mixture is formed into a green body by extrusion and optionally drying.
The resulting green body is next fired in air to temperatures of 1000xc2x0 C.-1500xc2x0 C., preferably between about 1200xc2x0 C.-1400xc2x0 C., with a soak time in that temperature range of a duration sufficient to form a sintered body, the soak time being preferably about 1 to 10 hours, more preferably 2 to 6 hours.
Composite cordierite bodies provided in accordance with the present invention are especially suited in applications of filtering diesel exhaust.